A molecular thermodynamic framework for the representation of the solubilities of several binary systems, amino acids-water (nine systems) and peptides-water (five systems), has been developed and tested. The proposed model takes into account simultaneously the chemical and physical equilibrium. The latter is described by the UNIQUAC model combined with a Debye-Hiickel term. Molal activity coefficient data from the literature were used in order to estimate the new UNIQUAC parameters. These parameters, together with solubility data, were the basis for the correlation of the solubility product of the different amino acids considered in this work. With this approach, very satisfactory results have been obtained for the representation and prediction of the solubilities of amino acids in water as a function of both temperature and pH. lNTRODUCTION Knowledge of solid-liquid equilibrium data is essential for the design, optimization and scale-up of separation processes handling with solids. In industrial processes, large numbers of solid and liquid mixtures exist, but only few experimental data are available for the required systems. Furthermore, the vapor-liquid and liquid-liquid equilibrium data do not provide information directty useful for predicting solid-liquid equilibria due to the fairly different temperature ranges of each phase equilibrium, so it is necessary to extrapolate or to estimate the equilibria from appropriate correlations. It is, therefore, valuable to develop new models for predicting solid-liquid equilibria. The properties of aqueous systems containing salts and large charged molecules (Prausnitz, 1989) are particularly important for the biotechnology industry. The study of amino acids solubility in water is a good starting point for the understanding of biochemical systems, due to the many similarities that they have with others biomolecules (Orella and Kirwan, 1991). In this way, Nass (1988), Chen et al. (1989) and Gupta and Heidemann (1990), and more recently Pinho et al. (1994), among others, have contributed to the progress of the representation of amino acids and/or peptides solubilities, proposing different models and approaches. In this paper we propose a new model to describe the solid-liquid equilibria. Similarly to the model proposed by Pinho et al. (1994), our model combines chemical and physical equilibrium simultaneously. The modified molecular UNIQUAC model is used to 'Author to whom correspondence should be addressed.